Long term evolution (LTE) technology is a communication system standard made by the 3rd generation partnership project (3GPP), and the complete specification has been completed and finalized after years of development. Because of the advantages of less transmission delay, higher data transmission rate, improved system throughput, and flexible and efficient use of spectrum, etc., it is highly expected that LTE technology will be able to meet the users' demands for high rate and high immediacy in the next decade. To achieve these goals, orthogonal frequency division multiple access (OFDMA) is selected as the technology for downlink transmission of the base station of LTE. However, OFDMA transmission consumes more energy and therefore is not suitable for user equipment (UE) with limited power. In other words, the OFDMA technology is not suitable for uplink transmission.
Therefore, in the LTE standard, single carrier frequency division multiple access (SC-FDMA) is used for uplink transmission. The difference between SC-FDMA and OFDMA is that: for SC-FDMA, discrete Fourier transform (DFT) is performed before inverse fast Fourier transform (IFFT), and after performing DFT, data symbol will be spread over all subcarriers to generate a virtual single-frequency structure. This structure is also called DFT-spread OFDM. Through this structure, SC-FDMA can have a peak to average power ration (PAPR) lower than OFDMA. Thus, the efficiency of power utilization would be improved when the user equipment performs uplink transmission, thereby extending battery life.
In the LTE system, the spectrum that can be used by LTE is divided into multiple resource blocks (RB), and one resource block is the smallest unit of resource allocation of LTE. Each resource block occupies a frequency band of 180 kHz in terms of the frequency domain. This frequency band includes 12 consecutive subcarriers, and each resource block includes a transmission time interval (TTI) in the unit of one millisecond in terms of the time domain. In this disclosure, the data bits that one resource block can carry is called “resource block capacity.”
In the downlink implemented by OFDMA, the base station usually allocates resource blocks to the user equipment having the best channel quality, so as to achieve multi-user diversity and maximize the overall transmission rate. Therefore, the channel-dependent scheduling (CDS) algorithm is very suitable for the downlink.
In the uplink of LTE, however, due to the limitations of the SC-FDMA technology, allocation of the resource blocks to the user equipment must comply with the limitation of consecutiveness. Specifically, the resource blocks allocated to one user equipment have to be consecutive in the frequency band. This limitation of SC-FDMA significantly reduces the flexibility in resource allocation when the base station allocates the resource blocks to the user equipment. Here, this limitation is called “consecutive resource block allocation.” In addition, another limitation that affects the uplink transmission rate during resource allocation is that one user equipment must adopt the same modulation and coding scheme (MCS) in the resource block being allocated. Therefore, for the user equipment, the resource block capacity that can be achieved on each resource block being allocated has to be the smallest capacity in the resource blocks being allocated. Here, this limitation is called “Fixed MCS format.”
In the current research on LTE uplink resource allocation (e.g. recursive maximum expansion (RME) algorithm), usually “consecutive resource block allocation” is the only limitation that is taken into account, and the important limitation “fixed MCS format” is ignored. As a result, the overall transmission rate of the system becomes very unsatisfactory when “fixed MCS format” is taken into consideration.
According to the relevant literature, methods of LTE uplink resource allocation are generally categorized into two types. The first type is to allocate the resource blocks to the user equipment with better SNR (Signal to Noise Ratio). The second type is to allocate the resource blocks to the user equipment that can temporarily elevate the overall transmission rate of the system to the highest level. These methods both achieve favorable overall transmission rates in the first few resource block allocations. However, the influence of the current allocation to the subsequent allocations is not taken into account, and as a result, the final overall transmission rate is not ideal as expected. For example, the conventional base station is configured to allocate resource blocks to the next user equipment only after completing allocation of resource blocks to the current user equipment. For this reason, even though favorable transmission rates may be achieved in the first few user equipment, the transmission rates for the later user equipment may decrease significantly, which affects the overall LTE uplink transmission rate.